Fault current sensor device with radio transceiver

ABSTRACT

A fault current sensor device detects and distinguishes abnormal current events on alternating current overhead and underground power transmission lines. The sensor distinguishes whether the momentary or sustained fault is a line-to-ground fault, line-to-line fault or a three-phase fault. The sensor determines whether the overload has occurred on all three phases, or only on one or two phases, of the power line in an unbalanced situation. The device can be remotely reprogrammed to alter its trigger or threshold level and can be remotely reset after a fault has occurred.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power transmission and distributionsystems, more particularly to devices for detecting and transmittingfault current information from power transmission and distribution linesto a control center, switching center or other designated groundstation.

2. State of the Art

In the field of power transmission and distribution, generating systemsproduce electrical power which is transmitted through a grid ofelectrical high voltage alternating-current (AC), three-phase powerlines. Occasionally, a transmission or distribution power lineexperiences a fault in which, for example, a short circuit or equipmentfailure on a power line causes a circuit breaker to trip open, causing apower interruption to the customer. Early detection and characterizationof these faults in an electrical power transmission and distributionsystem are essential to a quick resolution of the problem and to futureplanning of upgrades to the power transmission and distribution system.

Prior art fault sensors or devices can only sense and record abnormalcurrent that causes a sustained outage. Most often the device needs tobe manually reset at the device's location. In addition, these prior artdevices are not provided with data transmission capability. To identifythe power line section with a fault, each prior art device needs to bevisually inspected, which often takes hours to complete and prolongsoutage time for the affected customers.

Retaining sensor integrity over the life of the sensor is key tominimizing operation and maintenance cost. In the prior art, in order tomonitor line current and transmit information relating to the faultsoccurring on a transmission or distribution line, batteries were neededto power a microprocessor and transmitter. The problem arises with suchprior art devices that the batteries became exhausted over a period oftime and require replacement or recharging. To manage such batterymaintenance and/or replacement at thousands of remote sensing locationshas involved considerable expense and has often resulted in a failure todetect faults over significant periods of time.

Other prior art devices mounted on an overhead transmission ordistribution line derive power by utilizing the energy stored in themagnetic field surrounding the operating conductor. At locations such asnear the end of a distribution line, the load current may be at such alow level that there is not enough energy to power the fault sensor.Such prior art devices utilize a magnetic iron collar surrounding thetransmission or distribution line for extracting the magnetic fieldenergy as shown in U.S. Pat. Nos. 4,635,055, 4,728,887 and 4,808,917.These collar devices are relatively bulky, expensive, heavy anddifficult to install. Moreover, for satisfactory operation, it isnecessary for the iron magnetic collar to be completely closed aroundthe conductor without any gap, to provide flux continuity around thetransmission line. This closed collar arrangement is necessary both inorder to derive adequate power and also to isolate the measurement ofcurrent from the effect of other nearby conductors. Such closed ironcore clamp devices are heavy and difficult to install, requiring specialtools and in some cases two workers.

Another problem with the prior art fault sensors is that they are notremotely programmable. Thus, programming instructions and/orcalibrations made prior to installation cannot be changed withoutretrieving or removing the device from the power line.

It is therefore one object of the present invention to provide a sensingdevice that can be attached to a transmission or distribution power lineand which will sense and transmit at least one alarm condition usingcircuitry which is operated by power not dependent on power flowing inthe overhead transmission or distribution power line.

Another object of the invention is to provide an overhead sensing devicefor a power line that can be attached to an overhead power line by asimple clamp on a shotgun hot stick, measure at least one power lineoperating characteristics, and transmit any fault information and/oralarm condition to a designated ground station using a transceiverpowered by a bank of double-layer capacitors.

Another object of the invention is to provide an underground sensingdevice that can be clamped on the power line and that can measure atleast one operating characteristic and then transmit any faultinformation and/or alarm condition to a designated ground station usinga transceiver and a fiber optic link.

Another object of the invention is to provide a sensing device for anoverhead power line that can be completely controlled or programmed bycommunication signals from the ground using a transceiver providedwithin the device.

Another object of the invention is to provide a compact, lightweightsensing device for an overhead power line that can be easily attachedand removed from a power line and measure at least one of its operatingcharacteristics.

Still another object of the invention is to provide a sensing device fora power line that can be attached to a power line for measuring at leastone of its operating characteristics and that is particularly welladapted for economy of manufacture.

SUMMARY OF THE INVENTION

The aforesaid and other objects of the invention are accomplished by thesensing device of the present invention. The sensing device may beeither an overhead or underground sensing device. The overhead sensingdevice is comprised of an elongated housing formed from extruded orotherwise processed plastic. Fixed to the housing is a clamp assemblyhaving a jaw portion adapted to extend at least partially around thecircumference of a power line and compatible with standard industryinstallation tools. The sensing device can detect and distinguishbetween various types of faults on transmission and distribution lines,preferably including momentary outage, sustained outage, normaloverload, or inrush. The device only monitors current when a faultoccurs. A pickup current sensing coil is housed at a fixed distance fromthe power line and measures the magnetic field strength at that point,which is proportional to the power line current. When a fault currentevent is detected, the signal is sent through a diode and then on to amicroprocessor situated within the housing. The signal is processed bythe microprocessor, which distinguishes which type of fault current hasoccurred. Information concerning the type of fault current istransmitted by a transceiver connected to an antenna within the housingin the overhead sensor device.

In an underground sensor device, the antenna is located on an equipmentvault and is connected to the sensor device by a fiber optic link.Signals from the sensor are transmitted via the antenna and received andprocessed by monitoring equipment at a remotely located ground station.

If the device has alarmed because of fault current conditions, then whennormal current levels return the device can reset itself immediately orreset itself after a predetermined period of time, or be reset remotelyfrom an operating center or designated ground station. Power foractivating the sensor circuitry and for processing and transmittingrelated signals is furnished by a bank of double-layer capacitors (whichhave typical values greater than one farad). This bank is recharged byphotovoltaic cells attached to the outer surface of the overhead sensordevice or by a current transformer surrounding the underground powerline.

Central circuitry for the transceiver circuit includes a microprocessorwhich can be initially programmed to provide trigger and thresholdlevels. The microprocessor can also be controlled from the remote groundstation to alter the operating program of the device if different levelsare desired. A time delay for resetting the device can also bereprogrammed remotely.

An important feature of the invention is the provision of a wakeupcircuit for detecting the occurrance of a fault condition and generatingin response thereto a wakeup signal. The microprocessor is responsive tothe wakeup signal for exiting a sleep mode and is responsive to asensor-derived digital signal for detecting an abnormal condition andthereafter distinguishing whether any of a number of different types offaults has occurred. Since the device is in sleep mode a preponderanceof the time, power requirements are minimized.

With the information provided by the device, operators can respond tofaults promptly and appropriately. For a sustained alarm, operators maychoose to isolate the affected section and perform load switching torestore service to affected customers; for an overload alarm, operatorsmay execute load transfers to relieve the overloaded line section; for amomentary outage alarm, operators may dispatch line patrolmen toidentify the possible cause of the trouble, such as tree branchesgetting too close to the power line, and schedule a maintenance crew tocorrect the problem; for an inrush alarm, operators may refer theproblem to engineers, who may then devise remedial action to correct thedeficiency.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be further understood from the followingdescription in conjunction with the appending drawing. In the drawing:

FIG. 1 is a front elevational view of an overhead fault sensor, inaccordance with one embodiment of the invention, mounted on a powerline;

FIG. 2 is a view in end elevation of the overhead fault sensor device ofFIG. 1;

FIG. 3 is an illustration of the overhead fault sensor being mounted ona power line;

FIG. 4A is a block diagram of the principal components of the overheadfault sensor of FIG. 1;

FIG. 4B is a block diagram of the principal components of an undergroundfault sensor;

FIG. 5A is a schematic diagram of the precision rectifier of the currentsensing means of the invention;

FIG. 5B is a schematic of a power supply and a rechargeable power backupsystem of the overhead fault sensor;

FIG. 5C is a schematic of a power supply and a rechargeable power backupsystem of the underground fault sensor;

FIG. 6 is a flow chart illustrating a logic flow of the fault sensingdevice used to distinguish between various fault conditions;

FIG. 7A is a front elevational view of an underground fault sensor, inaccordance with another embodiment of the invention;

FIG. 7B is a view in end elevation of the overhead fault sensor deviceof FIG. 7A;

FIG. 7C is an illustration of the underground fault sensor in a vault;

FIG. 7D is a front elevational view of a transceiver portion of theunderground fault sensor;

FIG. 7E is a view in end elevation of the transceiver portion of theunderground fault sensor;

FIG. 8 is an illustration of the multiple fault sensors in remotecommunication with a ground station; and

FIG. 9 is a block diagram of a fault sensor monitoring system inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

With reference to the drawings, FIG. 1 and FIG. 2 show an overhead faultsensor device 1 embodying principles of the present invention as thefault sensor device appears in a typical installation when attached toan overhead power line 2. When in use the device will detect occurrenceof, distinguish and transmit information concerning an alarm condition.The device senses line current under microprocessor control in a mannerlater described in relation to FIG. 4A, FIG. 4B, FIG. 5A, FIG. 5B andFIG. 5C.

A sensor identification number is preprogrammed into the sensor. Thisidentification number and alarm condition information are transmitted inreal time by a radio transceiver 3 so that the data can be received,recorded and processed at a remote designated ground station 29 such asa control center. The ground station may be equipped with suitable datareceiving and storage equipment for monitoring a collection of sensordevices attached to different transmission and distribution power linesat various distances, as well as means for sending signals back to eachsensor device for reprogramming it or otherwise controlling itsoperation.

In broad terms, the overhead sensor device, as shown in FIG. 1 and FIG.2, is comprised of an elongated housing 4 within which is containedelectronic circuitry required for its operation. The housing is made ofmolded or extruded plastic which is ultraviolet resistant, such as ABSplastic. A clamp assembly 5 is centrally located for holding the devicefirmly to an overhead power line 2. A dipole antenna 6 is located justunder the surface of the housing for sending via RF radio signals dataobtained by the sensors of the device concerning faults and power linecharacteristics and for receiving control or reprogramming signals sentback to the device from the remote ground monitoring and control station29 or other ground station. Power for operating the fault sensor issupplied by two panels of photovoltaic cells 7 attached to the outersurface of the housing and wired so as to charge a bank of double-layercapacitors 8 within the housing.

The clamp assembly 5 consists of the housing frame, a lower jaw and aneyescrew 9. As shown in FIG. 1, the lower jaw of the clamp assembly 5,preferably made of metal, moves as the eyescrew 9 is turned to clamp onto the overhead power line 2. Threads provided in the plastic housingfor the eyescrew are also metal. The length of the screw and size of thelower jaw of the clamp assembly 5 allow gripping various diameters ofoverhead power lines. A lock washer allows the lower jaw of the clampassembly 5 to remain firmly attached to the overhead power line 2.

As shown in FIG. 1 and FIG. 2, the equipment within the sensing deviceis primarily molded to the housing 4. A pickup current coil 12 is moldedat a precise location in the housing 4. A microprocessor 11 is shieldedby a metal foil and then molded into the housing 4. The transceiver 3 isalso molded inside. The two panels of photovoltaic cells 7 are bonded tothe housing 4. The clamp assembly 5 is molded into the housing 4.

As shown in FIG. 3, the fault sensor is mounted on an energized overheadpower line easily and quickly by means of a so-called "hot stick" 10manipulated by an individual on the ground, in a bucket track, or from apower line pole. The hot stick 10 includes a "shotgun" internalattachment mechanism which attaches to the eyescrew 9 of the overheadfault sensor 1. To attach the overhead fault sensor 1, the hot stick 10is attached first to the sensor. Then the sensor is mounted on theoverhead power line 2 and the handle of the hot stick 10 is turned,thereby attaching the device to the overhead power line 2. When theoverhead device 1 has been mounted, the shotgun internal attachmentmechanism is deactivated and the hot stick 10 is detached from thedevice. Reinserting the hot stick and turning it in the oppositedirection will open the clamp assembly 5 and allow the overhead faultsensor 1 to be removed from the overhead transmission or distributionpower line 2. This attachment and removal feature provides flexibilityand efficiency in placing the fault sensors in the transmission anddistribution system.

Referring to FIG. 4A, a block diagram for the overhead fault sensor isshown. Upon the occurence of a power line fault, a current is generatedfrom the current sensing coil 12 across a resistor 14. This signal issufficient enough to toggle the CMOS Schmitt trigger NAND gate 15 from alogic high state to a logic low state. This change in logic stategenerates an interrupt or wakeup signal to the microprocessor 11. Themicroprocessor 11 awakens, powers the electronics and begins samplingthe current value, through the precision rectifier 13. Themicroprocessor 11 is programmed to follow a logic flow chart to bedescribed in relation to FIG. 6. A power switcher 16 takes the voltagefrom the bank of double-layer capacitors 8 and regulates it to a setvoltage for the microprocessor 11 and to a prescribed voltage range forthe analog circuitry. The double-layer capacitor banks 8 are charged byphotovoltaic cells 7. When the type of fault is distinguished, then theradio transceiver 3 sends the sensor identification number and eventinformation to a load center or any other designated ground station viathe antenna 6.

Referring to FIG. 4B, a block diagram for the underground fault sensor17 is shown. Upon the occurence of a power line fault, a current isgenerated from the current sensing coil 12 across a resistor 14. Thissignal is sufficient enough to toggle the CMOS Schmitt trigger NAND gate15 from a logic high state to a logic low state. This change in logicstate generates an interrupt or wakeup signal to the microprocessor 11.The microprocessor 11 awakens, powers the electronics and beginssampling the current value through the precision rectifier 13. Themicroprocessor 11 is programmed to follow a logic flow chart to bedescribed in relation to FIG. 6. The power switcher 16 takes the voltagefrom the bank of double-layer capacitors 8 and regulates it to fivevolts for the microprocessor 11. The double-layer capacitor bank 8 ispowered by a current transformer 18 which is clamped around theunderground power line 2. When the type of fault is distinguished, thenthe microprocessor 11 sends the sensor identification number and eventinformation through a fiber optic link 19 to a transceiver 3 which has apatch antenna 20. The radio transceiver 3 is powered by a battery 21.The signal is then sent to a load center or any other designated groundstation.

With reference to FIG. 4A, FIG. 4B, and FIG. 5A, the fault sensordevice's electronics include a microprocessor 11, RAM, I/O circuitry,bipolar voltage limiter, timer components, an A/D converter 22,capacitors, a CMOS Schmitt trigger NAND gate 15 and a precisionrectifier 13. Sampled values of the monitored parameters are digitizedby the A/D converter 22, stored in RAM, processed by the microprocessor11 in accordance with instructions stored within an EEPROM board, andthen sent over an eight-byte databus to a parallel spread-spectrumtransceiver 3. The ground station at a designated location includes amicroprocessor (similar to microprocessor 11) to which signals receivedfrom the devices are supplied for further processing, such asdetermining where the fault has occurred. Then the data are communicatedto a central data receiving and control facility by a data linkschematically indicated in FIG. 8, such as a Supervisory Control andData Acquisition (SCADA) remote terminal unit (RTU) link 23 or radio.This aspect of the invention is described in greater detail below.

With reference to FIG. 4A and FIG. 5B, current flow through the powerline is measured by a current sensing pickup coil 12 which is located ata fixed distance from the power line. A current is induced in the coil12 by the magnetic fields of the power line 2. When a fault conditionoccurs, a current is generated from the current sensing coil 12 acrossthe resistor 14. This toggles the CMOS Schmitt trigger NAND gate 15 froma logic high state to a logic low state which wakes up themicroprocessor 11. The microprocessor 11 powers the electronics andtakes a current value reading through the precision rectifier 13. Theanalog signal is fed into the AID converter 22. The digitized signal isthen sent to the microprocessor 11 for processing, and then the finalinformation is sent on to the transceiver 3.

With reference to FIG. 5B and FIG. 4A, power to operate the overheadsensing device electronics and transceiver is derived from a bank ofdouble-layer capacitors 8. The bank of double-layer capacitors 8 ischarged by two panels of photovoltaic cells 7 mounted on the housingsurface 4 of the overhead fault sensing device. The photovoltaic cellpanels 7 are connected through a blocking diode 26. Each panel mayconsist of 21 cells in a series configuration, each cell being rated at25 mA at 0.5V for total voltage of 10.5 volts. Such cells measuring0.5×2.0 cm are commercially available. The photovoltaic cells 7 areconnected to the bank of double-layer capacitors 8. The bank ofdouble-layer capacitors 8 is connected to the power switcher 16. Anotherlead to the power switcher 16 connects to the microprocessor 11 andelectronics.

Two leads from the precision rectifier 13 of the current sensor 12connect to the A/D converter 22 on the microprocessor 11 (FIG. 4A). Bothof these leads have a prescribed voltage signal range. From the powerswitcher 16 there are two leads. One is a lead through a transistor 24which is switched off in the sleep mode until a fault condition hasoccurred or when there is a system functionality check. This lead isconnected to both the radio transceiver 3 and an I/O port 25 of themicroprocessor 11 through the transistor 24. The second lead providespower to the microprocessor 11.

Referring to FIG. 4B and FIG. 5C, power to operate the underground faultsensor 17 is derived from the bank of double-layer capacitors 8. Thebank of double-layer capacitors 8 is charged by inductive power fromcurrent through the underground power line 2. When there is minimalunderground power line current then the bank of double-layer capacitors8 is used. A hinged current transformer 18 with the power line 2 formingthe single turn primarily supplies all internal power to the undergroundfault sensing device 17. Current in the power line 2 induces voltage andcurrent in the windings of the current transformer 18 which is rectifiedby a bridge rectifier 27 as further described in U.S. Pat. No.4,886,980, incorporated herein by reference. The hinged currenttransformer 18 is connected to the bank of double-layer capacitors 8.The bank of double-layer capacitors 8 is connected to the power switcher16. Another lead to the power switcher 16 goes to the microprocessor 11and electronics. Two leads from the precision rectifier 13 of thecurrent sensor 12 connect to the A/D converter 22 on the microprocessor11. Both of these leads have a prescribed voltage range. From the powerswitcher 16 there is one lead which provides power to the microprocessor11. The fiber optic link 19 from the microprocessor 11 is switched offin the sleep mode until a fault condition has occurred or when there isa system functionality check. This fiber optic link 19 is connected tothe radio transceiver 3, which has a wakeup circuit. The radiotransceiver 3 is powered by a battery 21.

Operation of the fault sensor when attached to a power line 2 beingmonitored in a typical installation may be described, relative to FIG.4A and FIG. 4B, as follows.

The device is designed to activate the transceiver 3 when a fault occurson the power line 2. During "sleep mode," power is not applied to any ofthe circuit elements except the CMOS Schmitt trigger NAND gate (whichdraws about 0.1 mA), and when a power-up signal is generated, allelements of the circuit (in particular the transceiver 3 and themicroprocessor 11) are switched to an "on" condition. The transceiver 3is also powered up at timed intervals to check the status andoperability of the fault sensor device. During this time, reprogrammingsignals can be sent to the device from a controller, switching center ora ground station 29 at a designated location. In the underground faultsensor 17 the signals are sent to and from the microprocessor 11 througha fiber optic cable to and from the transceiver 3, which has a patchantenna 20 located on a vault (shown in greater detail in FIG. 7A andFIG. 7C.)

Referring to FIG. 6, the fault sensor activates when the power linecurrent exceeds the trigger or threshold current setting of the currentsensing coil 12 at time =0. The current sensor 12 waits for a second(time=1 second) to check whether the current goes to zero. If thecurrent has gone to zero, then the monitored line has experienced aninterruption or outage. The current sensor 12 waits for another 60seconds (time=61 seconds) to check whether the current remains zero. Ifthe current remains at zero, then the power line has experienced asustained outage. If the current does not remain at zero, then the linehas experienced a momentary interruption.

If only one phase senses the excessive current, the fault is aline-to-ground fault. If the sensors on two of the phases sense the highcurrent, the fault is a line-to-line fault. If the sensors on all threephases sense the high current, the fault is a three-phase fault. Thisdetective scheme is based on the operation of protective devices onpower lines. When a protective device detects a fault condition on thepower line, the protective device opens to de-energize the power linewithin one second and then typically recloses in 5 seconds to test theline to see if fault current condition still persists. This test is doneto prevent unnecessary prolonged outages caused by events such as treebranches momentarily coming in contact with the power line. If thecircuit test is successful, the circuit is reset back to normal. If thefault current condition persists, then the protective device will opento de-energize the line again. Typically, protective devices will testthe circuit three times within a 30 second period. At the end of thisperiod, if the fault current condition persists, the device will lockopen to de-energize the circuit until the affected section is isolated.

Therefore, at time=1 second, if the line current does not go to zero,the power line has not experienced any interruption. At time=61 seconds,if the line current is less than 90% of the threshold setting of thecurrent sensor, the power line has experienced an inrush currentprobably caused by the starting of a large motor. But at time=61seconds, if the line current is greater than 90% of the thresholdsettings, the line is most likely overloaded.

With reference to FIG. 7A and FIG. 7B, the underground fault sensordevice 17 is capable of sensing the current values with respect to thepower line 2 upon which the sensor device is mounted. A current sensorof the type previously described in relation to FIG. 5A is incorporatedin the underground fault sensor device 17. The power system of theunderground fault sensor device, with its current transformer 18 andbank of double-layer capacitors 8, has been previously described inrelation to FIG. 5C.

In broad terms, the underground sensor device 17 is comprised of anelongated housing 4 within which are contained elements of an electroniccircuit required for its operation. The housing 4 is made of molded orextruded plastic. The elongated housing is shaped like a "clam" with ahinge 271 and a clasp 28 which clamps onto the power line 2. Asillustrated in FIG. 7C, a fiber optic cable 19 is connected to thetransceiver 3, which has a patch antenna 20. As further shown in FIG. 7Dand FIG. 7E, the transceiver 3 is mounted on the vault cover with thepatch antenna 20.

With reference to FIG. 8 and FIG. 9, the overhead and underground faultsensor devices 1 and 17 are capable of both receiving and transmittingsignals, as well as sensing the values of various parameters associatedwith the respective power line 2 upon which the sensor device ismounted. Although other types of communications links may be utilized,the invention is described herein as comprising of RF transmitting andreceiving means in each of the fault sensors and in a ground station ata designated location 29. A ground station communication equipmentantenna 30 is used to transmit and receive information. All faultsensors transmit data on a single frequency channel for reception by theground station antenna 30. Signals from the ground station antenna 30are transmitted on a second channel for reception by the fault sensortransceiver 3 in a manner similar to that described in U.S. Pat. No.4,855,671, incorporated herein by reference.

Each of the devices is equipped to measure the absolute value of currentand may, if desired, be further equipped to measure other parameters.When a fault occurs on a particular power line 2 which may have severalsensing devices, several signals will be sent to provide the alarminformation. In order to reduce the probability of data collision, eachsensing device is programmed individually to stagger the alarm report.The staggered transmission algorithm is based on the sensing device'sidentification.

FIG. 8 illustrates how the sensors interact with the load center or anydesignated ground station 29. It is realistic to expect that up to ahundred sensor devices can communicate with one ground station 29 at adesignated location. As illustrated in FIG. 9, at the designatedlocation there is an antenna 30 with either a transceiver or a SCADA RTU23 which receives the information from the devices. The information itreceives includes the sensor identification, and alarm or faultcondition. The ground station 29 will also receive a sensor devicestatus report on a predetermined regular schedule from each sensordevice as to the device's functionality. During this reporting interval,the sensor device can also be reprogrammed.

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential character thereof. The presentlydisclosed embodiments are therefore considered in all respects to beillustrative and not restrictive. The scope of the invention isindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of equivalentsthereof are intended to be embraced therein.

What is claimed is:
 1. A method of distinguishing between a plurality ofdifferent types of faults using a fault sensor that monitors current ofan AC power transmission or distribution line, comprising the stepsof:a) detecting when the current exceeds a first predetermined thresholdlevel; b) at a first predetermined time after the current has beendetected as exceeding the predetermined threshold level, checking to seeif said current is zero; c) if the current at said first predeterminedtime is not zero, checking to see if the current is less than a secondpredetermined threshold level; d) if the current at said firstpredetermined time is zero, at a second predetermined time after thecurrent has been detected as exceeding the predetermined thresholdlevel, checking to see if the current is zero; e) if in Step c) thecurrent is less than the second predetermined threshold level,indicating a current inrush type of fault; f) if in Step c) the currentis not less than the second predetermined threshold level, indicating acurrent overload type of fault; g) if in Step d) the current is zero,indicating a sustained current outage type of fault and furtheridentifying the fault as being one of a line-to-ground fault, aline-to-line fault, or a three phase fault; h) if in Step d) the currentis not zero, indicating a momentary current outage type of fault andidentifying the fault as one of a line-to-ground fault, a line-to-linefault or a three phase fault.